The hematopoietic system has served as a paradigm for studying many aspects of developmental biology and for understanding adult stem cell biology. The system is no less useful for studying the process of aging, as the tremendous demands on hematopoiesis to produce numerous cells over the life-time of an animal render it sensitive to environmental or genetic factors that impact on cellular turnover. Studies of aging hematopoietic stem cells (HSCs) have repeatedly demonstrated that HSCs become less functional with age and their properties are altered. However, how the behavior of individual clones is affected over time or contributes to tissue turnover remains poorly understood. Here, we propose to generate a series of tools to study tissue turnover in young and aging mice. The first of the tools is based on the powerful Brainbow system for stochastically marking individual cells inducibly and uniquely. We will adapt this for the hematopoietic system by generating a series of reporters that will be specifically expressed in hematopoietic cells (Hemebow mice). The second set of tools will allow us to track these marked cells when emanating either from HSCs or from their differentiated progeny. This will be accomplished by generating inducible Cre drivers that are specifically expressed in HSC, or specifically expressed in differentiated hematopoietic cells. When the Hemebow reporter mice are used in combination with the specific Cre drivers, we will be able to track the progeny of HSCs or of progenitors in young or aged mice. When this marking system is combined with mutant mice which exhibit altered cellular regeneration properties with age, such as some p53 mouse mutants, we will be able to finely dissect tissue turnover in specific contexts. Thus, these tools will be broadly useful for tracking cell turnover and clonal dynamics in the aging hematopoietic system. PUBLIC HEALTH RELEVANCE: The way tissue regeneration changes with age is still poorly understood, partly because we lack precise tools to measure the rate of tissue turn-over. We propose here to generate two sets of engineered mice that will allow us to precisely track the regeneration of the blood system over time. These studies will be of use to many investigators studying aging as well as those in other fields.